1. Technical Field
The present invention relates generally to a slot antenna and, more particularly, to a low profile dual polarization slot array antenna which is capable of providing dual circular or linear polarization radiation with optimum efficiency and bandwidth.
2. Discussion
Direct communication systems commonly employ antennas for transmitting and receiving radiating energy between remote locations. Currently, antennas are widely employed for an increasing number of applications, many of which require a low profile, wide bandwidth antenna that can operate with polarized radiating energy. For example, advanced Direct Broadcast Systems (DBS) have been and are still being developed for future generation cable television transmission. Currently, North America Direct Broadcast Systems are being developed which transmit circular polarized (CP) energy. According to current specifications, these broadcast systems require low cost dual circular polarization eighteen inch aperture antennas at remote television locations for receiving the circular polarized radiating signals via satellite transponders.
In the past, conventional reflector antennas were commonly used which typically consisted of a reflector operatively coupled to a feed horn (polarizer) via a strut and an associated mounting structure. Such antennas include a Cassegrain antenna in which the feed horn is displaced from the reflector at a focal point on the front side thereof. However, such conventional reflector antennas generally occupy a relatively large volume and are easily susceptible to damage from the environment.
Low profile antenna concepts have been developed which include planar slot antennas. One type of slot antenna includes a double-layer structure which forms two propagation layers. Double-layer slot antennas historically have included the excitation of a transverse-electromagnetic (TEM) mode travelling wave between a pair of parallel metallic plates. This type of slot antenna further involves radio frequency (RF) energy leakage through radiating slots formed on the upper metallic plate so as to form a boresight pencil beam. Such slot antennas have generally exhibited a relatively simple mechanical structure with potentially low fabrication costs. However, there are recognized limitations associated with the conventional slot antenna approaches. These limitations include the fact that either single feed designs or overly complicated multiple feed designs are generally employed to excite a pure TEM mode travelling wave between the parallel plates. While a number of feed design approaches have been proposed, the prior concepts are generally limited to a single polarization (CP or linear) or involve high complexity and exhibit low efficiency with a relatively narrow bandwidth.
Another type of slot antenna includes a radial line slot array antenna which has either a single or double layer structure with a plurality of coupling slots formed along a spiral pattern. An example of one such radial line slot antenna is described in U.S. Pat. No. 5,175,561 issued to Goto. Such single-layer slot antennas have been employed for Direct Broadcast Systems in Japan and are generally capable of operating with single polarization energy only. That is, the radial line slot array may handle only either right hand or left hand circular polarization. An additional feed on another layer could be added to the single layer radial line slot array to provide dual circular polarization beams. However, the two beams would be dependent upon each other and optimization of one would degrade the other. That means if one circular polarized beam is optimized, then the other circular polarized beam will likely exhibit rather poor performance. As a consequence, the radial line slot array generally is not capable of effectively handling the combination of both right hand and left hand circular polarization, while achieving reasonably acceptable bandwidth and performance criteria.
More recently, a low profile planar dual circular polarization slot array antenna has been developed which is described in U.S. patent application Ser. No. 08/104,460, filed Aug. 9, 1993, and entitled "Slot-Coupled Fed Dual Circular Polarization TEM Mode Slot Array Antenna", now U.S. Pat. No. 5,467,100. The aforementioned allowed Patent Application is assigned to the assignee of the present invention and is hereby incorporated by reference. The above disclosed slot antenna has a low profile assembly with a pair of oppositely disposed metallic plates dielectrically separated therebetween. An array of radiating elements are formed on one plate while an array of coupling slots are formed on the other plate. A first beamforming feed network communicates with an array of horizontal coupling slots, while a second beamforming feed network communicates with a vertical array of coupling slots. While the aforementioned slot antenna realizes several advancements over the conventional antennas such as a low profile assembly and efficient operation, the present invention is capable of providing increased compactness, enhanced efficiency with minimal feed line interference, among other advantages.
It is therefore desirable to provide for a low profile planar dual polarization slot array antenna which overcomes limitations which may be associated with the above-mentioned prior art approaches. More particularly, it is desirable to provide for a low profile slot antenna which realizes minimal signal interference and has a low profile assembly. It is further desirable to provide for a double-layered slot antenna which is capable of operating with both right hand and left hand circular polarization and involves relatively low fabrication costs and less complexity, while maintaining high efficiency and wide bandwidth capabilities. In addition, it is further desirable to provide for such a slot antenna which exhibits two circular polarized beams which are optimized independent of one another.